EVOH is widely used for food packaging, because it has a high gas barrier property and excellent resistance properties against oil and organic solvents, aroma retention property, transparency, and the like. For food packaging, various kinds of molding processes are employed. Examples of such molding processes includes an extrusion molding process for films, a blow molding process for bottles, a vacuum molding process for various kinds of packages, and the like. Such molding processes start by feeding resin pellets into a hopper, etc. of an extruder.
However, there is a problem that conventional EVOH resin pellets tend to be blocked in the hopper when the resins are fed into the extruder, and thus the resins are not fed into the extruder smoothly.
Furthermore, since the melting temperature should be 200° C. or more at the time of the molding process, unless the polymer is provided with an improved thermal property, the polymer is deteriorated at the time of melt molding. As a result, fish eye or hard spots are formed, thus deteriorating the quality of the products. Therefore, it is necessary to provide the polymer with an improved thermal stability.
Hitherto, in order to provide EVOH with such a property, a method of adding a minor component such as an acid material and/or a metal salt, etc. is well known. For improving the long-run property and the appearance deterioration caused by the gel and hard spots, etc., for example, JP 64-66262A discloses an EVOH of a composition that contains 0.0005–0.05 weight % (based on metal) of a metal salt belonging to group II of the periodic table, 0.002–0.2 weight % of acid having a pKa of 3.5 or more and at boiling point of 180° C. or more, and 0.01–0.2 weight % of acid having a pKa of 3.5 or more and a boiling point of 120° C. or less and which exhibits a certain flow ability.
The following are examples of a well-known method for obtaining EVOH pellets to which the minor component is added as mentioned above.
(1) A method of spraying an aqueous solution of the minor component to EVOH pellets, blending thereof with a Henschel mixer, and then drying thereof (JP 55-12108A).
(2) A method of mixing a minor component in the form of a powder to EVOH pellets, and dry-blending thereof with a super mixer (JP57-34148A).
(3) A method of immersing EVOH pellets in an aqueous solution containing a minor component and removing water from EVOH pellets, followed by drying (JP64-66262A).
(4) A method of adjusting the water content of EVOH pellets to 20 to 80 weight %, and bringing the EVOH pellets into contact with an aqueous solution of at least one compound selected from the group consisting of a boron compound, acetate and a phosphoric acid compound (WO/99/05213A).
However, in the method explained in the above (1) or (2), the minor components cannot be contained in EVOH pellets sufficiently homogeneously, and it is difficult to control the added amount of the minor component. Thus, it is difficult to obtain products with a stable quality. Furthermore, the method explained in the above (3) or (4) is advantageous in that it is easy to control the amount of the minor component contained in the EVOH pellets by adjusting the concentration of the solution. However, when the EVOH pellets treated by these methods are molded by melt extrusion, the motor torque and torque variation of the extruder are increased. Therefore, in general, it is necessary to add a lubricant to EVOH resin composition at the time of extrusion. However, the EVOH resin composition is used preferably for the applications of food packaging, etc., and therefore, the use of such lubricants is not necessarily preferable from the sanitary viewpoint. Thus, it has been demanded that the lubricants be reduced or avoided.
The EVOH pellets treated by the method (3) or (4) are generally obtained as follows. A methanol solution of saponified EVOH is allowed to precipitate in the form of a strand in a coagulation bath of a water/methanol mixed solution. The obtained strand is cut into pellets. However, in the case of EVOH of an ethylene content of less than 20 mol % and/or EVOH having a saponification degree of less than 95 mol %, strands are not precipitated easily in the coagulation bath, and furthermore, cutting errors or contamination of fine powders easily occur, thus making it difficult to produce pellets stably. Occasionally, strands may not be precipitated, and paste-like EVOH is precipitated as a crumb-like product coagulated in an indeterminate shape. However, it is difficult to mix the minor component homogeneously into the crumb-like precipitates processed by the methods (3) or (4). Thus, it is not possible to obtain products of a stable quality.
On the other hand, even if EVOH of an ethylene content of 20 mol % or more and/or the saponification degree of 95 mol % or more is used, when the strands are precipitated at high speed in order to improve the productivity, strands are not precipitated stably. Thus, it is difficult to produce pellets stably. Consequently, it was not possible to obtain products of a stable quality by the method (3) or (4).
Furthermore, from the viewpoint of the production process, in the conventional method in which the EVOH pellets are immersed in a treating solution containing an acid material and/or a metal salt, a treating bath or a treating tower for the immersion treatment is required. The treating solution used in such a treating bath or treating tower is required to be thrown away after use or recycled in order to add an appropriate amount of acid materials and/or metal salts again, after the treatment of EVOH, because the acid materials and/or metal salts are consumed after the treatment.
It is not preferable to throw away the treating solution in view of the influence on environment. Therefore, in general, the treating solution is disposed of by way of a wastewater treating plant. Furthermore, when the treating solution is collected and reused, in order to obtain products having a stable quality, apparatus for adding acid materials and/or metal salts again and apparatus for removing impurities such as oligomer, etc. in the treating solution are required.
As mentioned above, the conventional methods suffer from a problem that large-scale treatment apparatus and a long treatment time are required, and that the production cost is high.
Furthermore, an example of a wet extrusion includes a melt extrusion molding performed at certain energy (JP11-58500A), and melt extrusion molding by controlling the temperature of cylinder to a certain temperature (JP 11-58501A).
However, the above-mentioned conventional methods require complex operations. Therefore, the methods are not preferable as a treating process for producing pellets of EVOH resin.
Furthermore, when pellets of EVOH resin are fed to an extruder, melted and then subjected to a molding process, if residue of a saponification catalyst contained in the resin at the time of production remains in the polymer, the thermal stability may be deteriorated.
Hitherto, in order to remove the residue of the saponification catalyst from the polymer, resin pellets are placed in a washing container and brought into contact in the solid state with a washing water (water) so as to diffuse and extract residue from the inside of the resin pellets to the outside (JP55-19242A).
However, the above-mentioned conventional method requires a long time and a large washing apparatus and space.
Furthermore, in the above-mentioned conventional method, a large amount of water is attached to the resin. Therefore, in order to eliminate the water, a hot-air drying using a dryer is required. In this case, there is a problem that pellets occasionally are melted and attached to each other depending upon the drying temperatures.
Furthermore, the conventional EVOH resin pellets are generally produced by a strand cutting method. That is, a strand (continuous stick-like product) is obtained by extruding a solution of resin from a metal mold into a poor solvent so as to precipitate, or a strand is obtained by extruding resins which are heated and melted and by cooling and solidifying thereof. Then, the obtained strand is cut into pellets having the constant size by using a strand cutter (JP 3-61507A). However, since the strand cutting method requires a process for continuously precipitating or cooling and solidifying the extruded resin, it is difficult to cut a large amount of resin with high accuracy in a short time. Therefore, there is a problem that strands cut off easily. In particular, EVOH of a low ethylene content or EVOH of a low saponification degree suffers from the above-mentioned problem significantly, because the precipitating rate for allowing the strand to be precipitated from the solution is slow.
Furthermore, when feeding water-containing EVOH resin pellets into the extruder, if a large amount of water is attached to the surface of the resin pellets, some problems arise. For example, pellets form bridges in the hopper; pellets are blocked in a feeder; water vapor is generated in the extruder, so that pellets of EVOH resin are melted and attached to each other; and the like. Specific problems include, bridges are formed in the hopper in the feeder, and thus raw materials are not fed to the extruder sufficiently. Another problem is that surface water separated in the feeder is fed to the extruder to generate water vapor at the lower part of the hopper, so that the raw material pellets are melted and attached to each other. As a result, the raw material resins are not fed to the extruder sufficiently. In particular, in this case of the water-containing EVOH, melting point of EVOH is lowered, the pellets are likely to be melted and attached to each other at the lower part of the hopper. Furthermore, insufficient feeding of the raw materials may change the feeding amount into the extruder. Thus, there arise some problems, for example, the adding amount of acid metal salt becomes unstable, deteriorating the thermal stability. Therefore, the improvement in methods for effectively feeding water-containing EVOH resin pellets into an extruder has been demanded.